Field of the Invention
The present invention generally relates to a pattern inspection apparatus and a pattern inspection method. More specifically, for example, it relates to an inspection apparatus that inspects a pattern by irradiating laser lights or electron beams so as to acquire an optical image of the pattern to be inspected, and to a method therefor.
Description of Related Art
In recent years, with the advance of high integration and large capacity of a large scale integrated circuit (LSI), the line width (critical dimension) required for circuits of semiconductor elements is becoming progressively narrower. Such semiconductor elements are manufactured by exposing and transferring a pattern onto a wafer to form a circuit by means of a reduced projection exposure apparatus, which is known as a stepper, by using an original or “master” pattern (also called a mask or a reticle, and will be generically referred to as a mask hereinafter) with a circuit pattern formed thereon. Then, in manufacturing the mask used for transferring such a fine circuit pattern onto a wafer, a pattern writing apparatus capable of writing or “drawing” fine circuit patterns by using electron beams needs to be employed. Pattern circuits may be written directly on the wafer by the pattern writing apparatus. Also, a laser beam writing apparatus that uses laser beams in place of electron beams for writing a pattern is under development.
Since LSI manufacturing requires a tremendous amount of manufacturing cost, it is crucial to improve its yield. However, as typified by a 1-gigabit DRAM (Dynamic Random Access Memory), the scale of a pattern configuring an LSI has been changing from on the order of submicron to nanometer. One of major factors that decrease the yield of the LSI manufacturing is a pattern defect of a mask used when exposing and transferring a fine pattern onto a semiconductor wafer by the photolithography technology. In recent years, with miniaturization of an LSI pattern formed on a semiconductor wafer, dimensions to be detected as a pattern defect have become extremely small. Thus, a pattern inspection apparatus for inspecting a defect of a transfer mask used in manufacturing LSI needs to be highly accurate.
Meanwhile, with development of multimedia technology, the size of LCD (Liquid Crystal Display) substrate is becoming larger, e.g., 500 mm×600 mm or greater, and the size of a pattern such as a TFT (Thin Film Transistor) or the like formed on the liquid crystal substrate is becoming finer. Therefore, it is increasingly required that an extremely small defect of a pattern should be inspected in a large range. For this reason, development of a pattern inspection apparatus that can efficiently and short-timely inspect a defect of a photomask used when manufacturing large area LCD patterns and large-area LCDs is urgently required.
As an inspection method, there is known a method of comparing an optical image of a pattern, formed on a target object or “sample”, such as a lithography mask, imaged at a predetermined magnification by using a magnifying optical system with design data or an optical image obtained by imaging the same pattern on the target object. For example, the following is known as pattern inspection methods: die-to-die inspection method that compares data of optical images of identical patterns at different positions on the same mask; and die-to-database inspection method that inputs, into the inspection apparatus, writing data (design pattern data) which is generated by converting pattern-designed CAD data to a writing apparatus specific format for input when writing a pattern on the mask, generates design image data (reference image) based on the input writing data, and compares the generated design image data with an optical image (serving as measurement data) obtained by imaging the pattern. According to the inspection method for use in such an inspection apparatus, a target object is placed on the stage so that a light flux may scan the object by the movement of the stage in order to perform an inspection. Specifically, the target object is irradiated with a light flux from the light source and the illumination optical system. Light transmitted through the target object or reflected therefrom is focused on a sensor through the optical system. An image captured by the sensor is transmitted as measurement data to the comparison circuit. In the comparison circuit, after performing position alignment of images, measurement data and reference data are compared with each other in accordance with an appropriate algorithm. If there is no matching between the compared data, it is determined that a pattern defect is present.
When performing the pattern inspection, the existence or nonexistence of a defect of a pattern in a stripe region is inspected by dividing the whole of an inspection region of a target object into a plurality of strip-shaped stripe regions and scanning a stripe region concerned in the longitudinal direction with inspection beams. In that case, inspection is performed in order for a next stripe region adjacent to the stripe region having been inspected. With the current trend of miniaturization of patterns, it is necessary to strengthen (increase) the light intensity of a deep ultraviolet (DUV) light used as an inspection light. For this reason, the stripe region having been inspected is heated by the scanning using by a laser light. Thus, if the same stripe region is inspected twice, characteristics of an image acquired for the first time and characteristics of an image acquired for the second time after the heating are completely different from each other. Such heat affects not only the stripe region concerned but also the adjacent stripe region. Usually, adjacent stripe regions are set to be mutually slightly overlapping so as to avoid omission of detection of a defect at the boundary. Therefore, when scanning a stripe region concerned to be inspected, a part of the adjacent stripe region is also scanned. Thus, there is a problem that, because of the influence of the heat, it becomes difficult to acquire a highly precise image even in inspecting an adjacent stripe region.
As pattern miniaturization further advances, since resolution limit may be exceeded when using a DUV light source. Therefore, an inspection apparatus that uses electron beams whose resolution is higher than that of a DUV light source will be needed. However, if a stripe region is scanned in the longitudinal direction with an electron beam, electron charge-up occurs in the stripe region having been inspected. Then, when the same stripe region is inspected twice, an image acquired for the first time and an image acquired for the second time after the charging up are completely different from each other. Such charge-up affects not only the stripe region concerned but also the adjacent stripe region. As described above, adjacent stripe regions are set to be mutually slightly overlapping so as to avoid omission of detection of a defect at the boundary. Therefore, when scanning a stripe region concerned to be inspected, a part of the adjacent stripe region is also scanned. Thus, there is a problem that, because of the influence of the charging up, it becomes difficult to acquire a highly precise image even in inspecting an adjacent stripe region.
As technique related to the inspection apparatus that performs inspection using electron beams, there is disclosed an inspection apparatus in which an image of one line in a stripe region concerned is obtained by performing scanning in a single stroke of an electron beam in the short side direction of the stripe region to be inspected (refer to, e.g., Japanese Patent Application Laid-open (JP-A) No. 2009-192345). The technique employed in this inspection apparatus is to repeat a line scanning back and forth for going scanning and returning scanning of the same line in order to perform image acquisition and perform discharging, etc. of the region having just been charged.
As mentioned above, there is a problem that the image of an adjacent stripe region is distorted by inspecting (scanning) a target stripe region. However, a method sufficient for solving this problem has not been established yet.